The invention relates to a method in a fiber web machine, in which method a fabric included in the fiber web machine is used for supporting a web produced by the fiber web machine, and in which method the location of the fabric is changed in the cross direction of the fiber web machine. The invention also relates to a method in a fiber web machine and to a software product.
In prior art, the fabrics of a fiber web machine are oscillated at a constant amplitude, or there is no oscillation at all. In the press section of the fiber web machine, the fabrics are press felts, which pass through at least one press nip. In oscillation, the fabric is run in the cross direction of the machine for example by means of one guide roll. In other words, the location of the fabric in the cross direction of the fiber web machine changes. The purpose of this is, for example, to prevent the uneven wear of the rolls that support the fabric. The guide roll is moved by means of actuators, which are provided with control devices to accomplish a motion of desired magnitude. In addition to the press section, fabrics can also be oscillated in the forming section which precedes the press section, and in the drying section which follows the press section. Irrespective of the application, at least some portion of the travel of the fabric supports the web produced by the fiber web machine.
In practice, the width of the fabric varies on the basis of the running situation, and the fabric also shifts in the cross direction of the fiber web machine. In other words, the location of the fabric with respect to the center line of the fiber web machine can vary. In addition, especially the press felt is deformed as it passes through the press nip. In this case, when the fabric is oscillated, but also without oscillation, there can be a random problem. In this problem, the edges of the web occasionally go to such an area of the fabric where the web has never been before or where the web has not been for a long period of time. The properties of an area which has not been used or which has been unused for a long period of time are different from the remaining areas of the fabric. In this case, a point of discontinuity is formed at the edges of the fabric, and the edge of the web is damaged on such a point of discontinuity. Especially in the press section, edge damage often results in a web break sooner or later.